The middle set of cylinders are the high pressure set, they exhaust into the front and rear set of cylinders with the front cylinders then using the front stack and the aft cylinders exhausting via a feed water heater through a small stack in the rear of the locomotive.Madman said:I am not familiar with the operation of the Tri-plex. In the video, I noticed a small stack at the rear of it's tender. What is it?